This invention concerns a device for controlling a hydraulic motor, particularly a device comprising a directional valve for directing hydraulic fluid from a pressure source to the motor, and a separate safety valve means for preventing hydraulic fluid under pressure from reaching the directional valve when activated.
The purpose of a control device of the above type including the safety valve is to accomplish an interruption of the hydraulic fluid supply to the directional valve and, thereby, a deactivation of the motor in emergency situations.
In previous devices of the above described type, as the one disclosed in U.S. Pat. No. 5,062,266, interruption of the hydraulic fluid supply to the directional valve is accomplished by opening up of a drain passage to a tank for discharging the pump pressure.
In some cases, however, this prior art type of safety means does not provide the intended safety function, namely at operation under low temperature conditions where the viscosity of the hydraulic fluid is increased. In such cases, a certain pressure tends to remain in the system, despite the opened drain passage. This means that there is a risk that the motor will still perform some movement after the safety means activation, which may be hazardous to people and or equipment.
The main object of the invention is to accomplish an improved device for controlling a hydraulic motor by which the features of the safety valve means ensure that there is no remaining pressure in the system and that no undesirable movement of the motor will occur after activation of the safety valve means.
Other objects and advantages of the invention will appear from the following description and claims.
Preferred embodiments of the invention are described below with reference to the accompanying drawings.